DE3885131T2 - Optical disc recording or reproducing apparatus and focus control systems therefor. - Google Patents

Description

The invention relates to an optical disk recording and/or reproducing device and to focus control systems for such a device.

Figures 1 and 2 of the accompanying drawings each show a circuit diagram of a previously proposed device for recording and/or reproducing with an optical disk and an optical disk as is to be used for a device according to Figure 1.

The optical disk recording and/or reproducing apparatus of Figure 1 is designed for recording and/or reproducing information from tracks formed on an optical disk 1. As is known, the optical disk 1 is mounted on a disk drive mechanism having a motor-driven shaft 2 driven by a spindle motor so as to drive the disk 1 in rotation. The optical disk to be used in the apparatus shown is, as shown in Figure 2, a pre-grooved type having a plurality of recording tracks defined by the pre-formed grooves G. As the optical disk to be used for recording, the embodiment shown may additionally use an optomagnetic disk. The disk 1 mounted on the spindle/shaft 2 is then driven at a predetermined constant speed.

An optical head is provided near the optical disk for optically reading or writing information in the recording tracks of the optical disk. The optical head 3 is connected to a radio frequency (RF) circuit 4. The RF circuit 4 converts, in the playback mode, the information read from the recording track by means of the optical head 3 into an electrical signal indicative of the read information. On the other hand, in the recording mode, the RF circuit 4 converts an electrical signal containing information into an optical information signal in a form recordable in the recording track.

The RF circuit 4 is connected to a signal processing circuit 5 that performs specific signal processing. The signal processing circuit 5 is connected to an input/output (I/O) circuit 6.

The RF circuit 4 also supplies a focus error signal to a focus servo circuit 7. The RF circuit 4 is further connected to a track servo circuit 8 to supply a track error signal thereto. One output of the focus servo circuit 8 is connected to the optical head 3. Another output of the focus servo circuit 7 is connected to a winding servo circuit 9 which controls the transverse displacement of the optical head 3.

The signal processing circuit 5, the focus servo circuit 7, the tracking servo circuit 8 and the winding servo circuit 9 are connected to a central processing unit (CPU) 10. This serves as a system controller. The CPU 10 supplies clock signals, timing signals, access signals and the like. The CPU 10 also supplies control signals to the above-mentioned respective circuits for controlling the operations thereof depending on the selected operation mode. The CPU 10 also serves to control the drive speed of the spindle motor, namely to control the rotation speed of the optical disk 1.

As shown in Figure 2, the grooves in the optical disk 1 are formed in a concentric circular or spiral shape. The grooves G are hereinafter referred to as "pre-grooves". Each pre-groove G has a width corresponding to Lamda/8 (where Lamda is the wavelength of the laser in the optical head). Adjacent pairs of pre-grooves G form a land area T therebetween, which serves as a recording track. As will be noted, the intensity of light reflected from the pre-grooves G and the land areas T is different. Based on the difference in the intensity reflected from the pre-grooves G and the land areas T, a tracking error signal is generated, so that a tracking servo system controls the optical head to project the light spot of the laser beam onto one of the desired tracks for tracking therealong.

In such an optical disk recording and playback device, the reflected light intensity changes frequently at any time when the laser beam spot passes over the pre-grooves during a seek operation, namely when the optical head is displaced across the tracks. The result is a high frequency signal St which is modulated by the pre-grooves. One such is shown in Figure 3(a) of the accompanying drawings. It is referred to hereinafter as the "crossing signal". It tends to be superimposed on the focus error signal.

The focus servo circuit 7, shown in detail in Figure 4 of the accompanying drawings, usually uses a phase compensation circuit 71 to increase the high frequency components of the focus error signal St so as to improve the response characteristics. The output signal of the phase compensation circuit 71 is supplied to a drive circuit 72. The drive circuit 72 generates a drive signal SFD for driving a focus actuator 73.

In such a circuit arrangement, namely, when the crossover signal is superimposed on the focus error signal St, the crossover signal may be amplified in the focus servo loop described above. The result is that the peak value of the amplified focus error signal tends to saturate, causing distortion of the waveform of the drive signal SFD, as shown in Figure 3b of the accompanying drawings. This distortion of the waveform of the drive signal SFD causes change/variation of the DC level. Such change/variation of the DC level tends to reduce the accuracy of the focus adjustment of the focus servo system.

According to one aspect of the present invention there is provided an optical disk recording and/or reproducing apparatus comprising:

an optical head for scanning a light beam across an optical disk having a plurality of generally circumferential grooves to reproduce an information signal including a focus error signal;

a focus servo system having a focus actuator operable to move an objective lens of the optical head to focus a light beam on the optical disk and

Means associated with the focus actuator for deriving a focus control signal based on the focus error signal and for controlling the focus actuator,

marked by:

Means active in the access mode of the device in which the light beam is translated across at least one of the grooves for removing a signal component modulated by said groove and superimposed on the focus error signal, said means for removing the signal component comprising a deadband circuit and a sample and hold circuit, said deadband circuit defining a deadband for the focus error signal for absorbing fluctuations of said focus error signal within the deadband, whereby a value held in said sample and hold circuit is unchanged.

According to another aspect of the present invention, there is provided an optical disk recording and/or reproducing apparatus, comprising:

an optical head for scanning a light beam across an optical disk having a plurality of generally circumferential grooves for reproducing an information signal including a focus error signal

a focus servo system having a focus actuator operable to move an objective lens of the optical head to focus a light beam on the optical disk; and

means associated with the focus actuator for deriving a focus control signal based on the focus error signal and controlling the focus actuator;

marked by:

Means active in the access mode of the device in which the light beam is translated across at least one of the grooves for removing a signal component modulated by said groove and superimposed on the focus error signal, said means for removing the signal component comprising a peak hold circuit for holding a peak value of the focus error signal, a base hold circuit for holding the base value of said focus error signal, and an adder circuit for adding the output signals of the peak and base hold circuits.

A preferred embodiment of the present invention will be described in more detail below. It is an optical disk recording and reproducing apparatus which effectively and stable for precise focus control can eliminate the influence of the crossing signal.

The preferred optical disk recording and playback device has a signal processing circuit in a focus servo circuit for suppressing the traverse signal component contained in the focus error signal. This component has a frequency determined by the pitch of the pre-provided grooves of the disk and by the speed of the transversely shifted laser beam. It modulates the focus error signal during a seek operation.

The signal processing circuit overcomes the influence of the crossing signal on the focus servo system, thus ensuring accurate focus control.

According to a further aspect of the present invention, there is provided a focus control system for an optical disk recording and/or reproducing apparatus including: an optical head for scanning a light beam across an optical disk having a plurality of generally circumferential grooves for reproducing an information signal including a focus error signal, and a focus servo system having a focus actuator operable to move an objective lens of the optical head to focus a light beam on the optical disk, the focus control system comprising:

a focus control signal generator associated with the focus actuator for deriving a focus control signal based on the focus error signal and for controlling the focus actuator,

marked by:

means for absorbing a crossover signal component, said means being arranged before said focus control signal generator for absorbing fluctuations of said focus error signal within a predetermined fluctuation range to eliminate a crossover signal superimposed on said focus error signal, said means for absorbing said signal component comprising a deadband circuit and a sample and hold circuit, said deadband circuit defining a deadband for said focus error signal for absorbing fluctuations of said focus error signal within said deadband, a value held in said sample and hold circuit being unchanged.

The preferred focus control system may include means for adjusting the deadband. The means for adjusting the deadband detects the level of the signal component to adjust the width of the deadband in response thereto. The means for adjusting the deadband may detect the signal component superimposed on an output signal of the sample and hold circuit to adjust the deadband.

In addition, the focus control system may also include means for forming a signal bypass path of said deadband circuit for feeding the input focus error signal directly to the sample and hold circuit, said signal path including a switch to be switched between a conductive state for forming said path and a non-conductive state for interrupting said path and for switching said conductive state in response to an indication signal of one of said tracking and focus adjustment states.

According to a still further aspect of the present invention, there is provided a focus control system for an optical disk recording and/or reproducing apparatus, said apparatus including: an optical head for scanning a light beam across an optical disk having a plurality of generally circumferential grooves to reproduce an information signal including a focus error signal, and a focus servo system having a focus actuator operable to move an objective lens of the optical head to focus a light beam on the optical disk, said focus control system comprising:

a focus control signal generator associated with the focus actuator for deriving a focus control signal based on the focus error signal and for controlling the focus actuator,

marked by,

a focus error signal smoothing means, said means being arranged before said focus control signal generator for smoothing said focus error signal so as to eliminate a crossover signal superimposed on said focus error signal, said focus error signal smoothing means having a peak value holding circuit for holding the peak value of said focus error signal, a base value holding circuit for holding the base value of this focus error signal and an adder circuit for adding the output signals of these peak and base hold circuits.

The peak and baseline hold circuits may each include a diode and the device may further include means for compensating for non-linear characteristics of the diodes.

The focus control system may also include means for forming a signal path as a bypass for the focus error signal smoothing means for feeding the input focus error signal directly to the output, said signal path comprising a switch operable between a conductive state for forming said path and a non-conductive state for interrupting said path and for switching on said conductive state dependent on an indicative signal of an operating state of the tracking and focus adjustment.

The invention will now be described by way of example with reference to the accompanying drawings, in which like parts have like reference numerals.

Figure 1 shows a schematic block diagram of a known optical disk recording and playback device;

Figure 2 is a plan view (with a partially enlarged cross-sectional view) of an optical disk of the pre-grooved type to be used in a recording and reproducing apparatus of Figure 1 as well as in the preferred embodiment of the invention;

Figures 3(a) and 3(b) show respective waveforms of the focus error signal and a drive signal in a focus servo system;

Figure 4 shows a block diagram of a focus servo circuit as used in a known device of Figure 1;

Fig. 5 is a circuit diagram showing a cross signal removing circuit and a control circuit which are a major part of an optical disk recording and reproducing apparatus according to a first embodiment of the invention;

Figure 6 is a graph showing the input/output characteristics of a deadband circuit associated with the circuit of Figure 5 for eliminating the crossing signal;

Figures 7(a) and 7(b) show waveforms of the focus error signal;

Figure 8 is a circuit diagram of another circuit for removing the crossover signal, this circuit to be used in an optical disk recording and reproducing apparatus according to another embodiment of the invention;

Figures 9(a) to 9(e) show waveforms of signals appearing at various points in the crossing signal elimination circuit of Figure 8;

Figures 10 and 11 show graphs of input/output characteristics of a deadband circuit associated with the circuit of Figure 8 for elimination of the crossing signal;

Figure 12 shows a waveform diagram of a focus error signal and an output signal of a sample and hold circuit used in the circuit of Figure 8 for eliminating the crossing signal ;

Figure 13 is a circuit diagram of another circuit for eliminating the crossover signal, the circuit being to be used in an optical disk recording and reproducing apparatus and embodying the invention;

Figures 14(a) and 14(b) show waveforms appearing in the circuit of Figure 13; and

Figure 15 shows a modification of the crossing signal elimination circuit of Figure 13.

Referring now to the figures and particularly to Figure 5, a crossover signal elimination circuit 11 is inserted between a high frequency circuit (such as the high frequency circuit 4 of Figure 1) and a phase compensation circuit (such a phase compensation circuit 71 as shown in Figure 4). The crossover signal elimination circuit 11 includes an operational amplifier 111 which forms a voltage follower circuit. The operational amplifier 111 has a non-inverting input terminal connected to the high frequency circuit (not shown) to receive a focus error signal St therefrom. The operational amplifier 111 also has an inverting input terminal connected to the output of the operational amplifier so that the voltage follower circuit is formed.

The output of the operational amplifier 111 is also connected to a deadband circuit 112. The deadband circuit 112 is connected to the phase compensation circuit (not shown) via a sample and hold circuit 116 comprising a resistor 113, a capacitor 114 and an operational amplifier 115, forming a voltage follower circuit.

The deadband circuit 112 includes a pair of diodes 117 and 118. The diodes 117 and 118 are arranged in parallel relation to each other and with opposite polarity. The deadband circuit 112 is designed to cancel the crossing signal superimposed on the focus error signal St by the potential difference of the forward or rising voltage (threshold voltage) of the diodes.

The held voltage VH of the sample and hold circuit 116 is normally supplied to the non-inverting input terminal of the operational amplifier 115. When the input voltage VFE of the dead band circuit 112 containing the superimposed crossover signal fluctuates within a fluctuation range (VH - VFE), the range being smaller than the rise voltage VD1 or VD2 or the diodes 117 and 188, the two diodes are kept off. The hold voltage VH of the sample and hold circuit 116 is therefore transferred to the phase compensation circuit.

On the other hand, if the range of the voltage variation (VH - VFE) is larger than the rise voltage of the diodes 117 and 118, both diodes will be conductive. The result is that the holding voltage VH is changed by an order of magnitude corresponding to a difference between the rise voltage VD1 or VD2 of the diodes 117 or 118 and the fluctuating voltage (VH - VFE).

It should be noted that in the arrangement described above, namely, when the input voltage VFE fluctuates with respect to the held voltage VH, the held voltage does not change as long as the voltage fluctuation is within a range determined by the rise voltages VD1 and VD2 of the diodes 117 and 118.

This voltage fluctuation range is hereinafter referred to as a "dead band". The crossing signal superimposed on the focus error signal St is successfully eliminated by using this dead band.

The input/output characteristics of the deadband circuit 112 are shown in Figure 6. As can be seen from Figure 6, the opposite connection of the diodes 117 and 118 causes a substantial absorption amount for low level signals, i.e. for signals in a range between VD1 and VD2.

Referring back to Figure 5, the deadband circuit 112 is associated with a control circuit 120 which controls the deadband circuit 112 between an active and inactive state. A switch 119 is provided to control the operating state of the deadband circuit. The control circuit 120 has input terminals 121 and 122 which are connected to a system controller (not shown). When a HIGH level input signal is present at one of the input terminals 121 and 122, the switch 119 is turned on (closed) via one of the two transistors 123 and 124 and via a differential amplifier 125 to make the deadband circuit 112 inactive.

The input level at input terminal 121 represents the operating state of the tracking servo system and is held HIGH while the recorder and reproducing apparatus is operating in the playback mode. This turns transistor 123 and hence switch 119 on to form a bypass circuit for passing the focus error signal through switch 119. Similarly, the input level of input terminal 122 represents the focus seek mode of operation. When focus seek is in operation, a HIGH level input signal is applied to input terminal 122. The result of this is that transistor 124 turns on to close switch 119. This forms the bypass circuit for passing the focus error signal St through switch 119.

On the other hand, when the optical head is moved across the tracks of the optical disk in the search mode, the tracking servo system turns off, thereby supplying a LOW level signal to the input terminal 121. As a result, the transistor 123 is turned off to turn off the switch 119. The bypass circuit is therefore interrupted by the switch 119 and the focus error signal St is supplied to the dead band circuit 112. Since the input/output characteristics of the dead band circuit 112 are as shown in Figure 6, at this time, the focus error signal St (shown in Figure 7(a)) has become the input signal of the Deadband circuit 112 superimposes the crossing signal as a component. This component is absorbed to produce a signal having the waveform shown in Figure 7(b). Since the output signal of deadband circuit 112 of Figure 7(b) has successfully eliminated the crossing signal component therefrom, driver circuit 72 of Figure 4 is not saturated by the output signal of phase compensation circuit 71.

On the other hand, when the recording and reproducing apparatus is turned on or when the focus servo is to be set, a focus search signal is supplied to the drive circuit 72 to move the object lens vertically for focus control. During focus search or adjustment, a HIGH level signal is supplied from the system controller to the input terminal 122. In response to the HIGH level signal at the input terminal 122, the transistor 124 becomes conductive. This raises the output level of the differential amplifier 125 to turn on the switch 119. This provides the above-mentioned bypass by means of the switch 119, namely to pass the focus error signal St to the phase compensation circuit. The focus adjustment can thus be carried out accurately and precisely.

In the above-mentioned first embodiment of the optical disk recording and reproducing apparatus, the dead band circuit is designed to be active only when the optical head is moved traversing the tracks of the optical disk, namely, to shift the laser beam spot across the predetermined grooves. The dead band circuit is active to absorb or cancel the traversing signal that may be superimposed on the focus error signal. In other words, since the dead band circuit is kept inactive during the operation of the track servo, this avoids the possibility of the focus servo system being brought into a defocused condition due to the influence of the dead band circuit.

Figure 8 shows another crossing signal elimination circuit according to a second embodiment of the invention. The crossing signal elimination circuit 222 has a deadband circuit 223 comprising a pair of diodes 217 and 218 and resistors 221 and 222 connected in series with a respective one of the diodes 217 and 218. As in the previous embodiment, the diodes 217 and 218 connected in parallel with each other with opposite polarity. Resistors 221 and 222 have respective resistances R₁ and R₂. A current source 225 is connected to a node between diode 217 and resistor 221. Similarly, a second current source 226 is connected to a node between diode 218 and resistor 222. Current sources 225 and 226 are arranged to supply respective currents I₁ and I₂. Currents I₁ and I₂ of current sources 225 and 226 serve to produce voltage drops VR1 and VR2 across resistors 221 and 222, namely to supply reset voltages (VFE - VR1, VFe + VR2), relative to input voltage FFE, to deadband circuit 223.

Diodes 217 and 218 therefore become conductive when the following conditions prevail:

VD1 smaller (VFE - VR1) - VH ..... (1)

VD2 small VH - (VFE + VR2) ..... (2)

The above conditions (1) and (2) may be changed into:

VD1 + VR1 small VFE - VH ..... (3)

-(VD2 + RR2) greater than VFE - VH ..... (4).

As can be seen from the above conditions, the diodes 217 and 218 are conductive when the amount of fluctuation of the input voltage VFE is outside the range defined by (VD1 + VR1) and (VD2 + VR2). When the diodes 217 and 218 become conductive, the held voltage VH of the sample and hold circuit 216 changes. In other words, as long as the amount of fluctuation of the input voltage VFE is kept within the range defined by (VD1 - VR1) and -(VD2 + VR2), the held voltage VH can be kept constant.

Since, as can be seen, the deadband is variable in a range defined by (VD1 - VR1) and (VD2 + VR2), namely depending on the voltage drop VR1 and VR2, the deadband can be adjusted by adjusting the currents I₁ and I₂ supplied by the current sources 225 and 226.

In the embodiment according to Figure 8, the current sources 225 and 226 are intended to change the output currents I₁ and I₂, namely depending on the level of the crossing signal. This adjusts the deadband range depending on the level of the crossing signal, namely to ensure the elimination of the crossing signal superimposed on the focus error signal. This avoids the level of the focus error signal being lowered excessively.

For this reason, a high-pass filter 232 comprising a capacitor 230 and a resistor 231 is connected to the output of the sample and hold circuit 216 to obtain the focus error output signal St1 therefrom. The high-pass filter 232 filters out the crossover signal SM1 from the focus error signal St1. The crossover signal SM1 filtered out by the high-pass filter 232 is supplied to a full-wave rectifying amplifier 233. The result is that when the crossover signal SM2 is superimposed on the focus error signal, which represents the input signal of the crossover signal elimination circuit 220 in Figure 9(a), and when the crossover signal SM2 causes the input signal variation to exceed the dead band defined by the dead band circuit 223, the excess amount of the crossover signal SM1 is filtered out by the high pass filter 232 and supplied to the full wave rectifying amplifier 233.

The full-wave rectifying amplifier 233 has an input resistor 234, a feedback resistor 235 and an operational amplifier 237 including a rectifying diode 236. The resistance values of the input resistor 234 and the feedback resistor 235 are set equal. With this circuit arrangement, the full-wave rectifying amplifier 233 provides a full-wave rectified output signal SMA as shown in Figure 9(c). The rectified output signal SMA of the full-wave rectifying amplifier 233 is fed via an operational amplifier 240 to an envelope detector circuit 243. This has a resistor 241 and a capacitor 242. As can be seen from Figure 8, the envelope detector circuit 243 has the structure of a low-pass filter. As a result of the envelope detector 243, an envelope signal SE (shown in Figure 9(d)) is obtained from the rectified output signal SMA derived from the crossing signal SM1 filtered out by the high-pass filter 232.

The high-pass filter 232, the full-wave rectifying amplifier 233, the operational amplifier 240 and the envelope detector 243 together form a crossing signal detector circuit.

The output of the envelope signal detector 243 is connected to an inverting amplifier 248. The inverting amplifier 248 has an operational amplifier 247, which in turn has an input resistor 245 and a feedback resistor 246. The input resistor 245 and the feedback resistor 246 have the same resistance value. In this arrangement, the inverting amplifier 248 receives the envelope signal SE supplied from the envelope detector 243 and inverts the received envelope signal to provide an output signal to the current sources 225 and 226. Assuming that the voltage level of the envelope signal SE supplied to the inverting amplifier 248 is Vc, the voltage level of the inverted envelope signal becomes -Vc.

The current source 225 comprises an operational amplifier 253 having an inverting input terminal, a non-inverting input terminal and an output terminal. A feedback resistor 250 is inserted between the non-inverting input terminal and the output terminal. Another feedback resistor 252 is inserted between the inverting input terminal and one end of an output resistor 251, the other end of which is connected to the output terminal of the operational amplifier 253. The non-inverting input terminal of the operational amplifier 253 is also connected via an input resistor 254 to a reference voltage source 257. This comprises a resistor 255 and a temperature compensation diode 256 to obtain a reference voltage VD3 therefrom. The inverting input terminal of the operational amplifier 253 is connected to the output of the inverting amplifier 248 through an input resistor 258, namely to obtain the inverted envelope signal. The output terminal of the operational amplifier 253 is connected to the resistor 221 of the dead band circuit 223 through the output resistor 251, namely to supply the current I₁. The resistance value of the output resistor 251 is set to a value equal to the resistance value R₁. of the resistor 221. On the other hand, the resistance values of the feedback resistors 250 and 252 and the input resistors 254 and 258 are selected to be equal to each other. With such a circuit arrangement, the following equation is obtained for the voltage appearing across the output resistor 251, namely with respect to the input voltages Vc and VD3 and the output current I₁:

I₁R₁ = Vc - VD3 ..... (5).

On the other hand, the voltage drop VR1 across the resistor 221 with respect to the current I₁ can be expressed by:

VR1 = R₁I₁ ..... (6).

The voltage drop VR1 can therefore be expressed as:

VR1 = Vc - VD3 ..... (7).

Following from the foregoing, the voltage VF1 defining the deadband and determined by the resistor 221 and the diode 217 can be expressed as:

VF1 = VD1 + VR1 = VD1 + Vc - VD3 ..... (8).

Assuming that the diodes 217, 218 and 256 are provided with equal rise voltage, the following equation can be derived from the previous equation (8):

VF1 = Vc ..... (9).

The voltage VF1 which defines the deadband and which is determined by the diode 217 and the resistor 221, can therefore be controlled proportionally to the output voltage Vc.

The current source 226 similarly has an operational amplifier 264 having a non-inverting input terminal, an inverting input terminal and an output terminal. A feedback resistor 260 is connected to the output terminal through an output resistor 261 at one end and to the non-inverting input terminal at the other end. Another feedback resistor 262 is arranged between the output terminal and the inverting input terminal. The non-inverting input terminal of the operational amplifier 264 is also connected to the reference voltage source 257 through an input resistor 265. The inverting input terminal is connected to the output of the inverting amplifier 248 to receive the inverted envelope signal -Vc therefrom through an input resistor 266. The Output resistor 261 has a resistance R₂ which is equal to that of resistor 222. The resistance values of feedback resistors 260 and 262 and input resistors 265 and 266 are chosen to be equal to one another.

With this circuit arrangement, the current source 256 produces a current I₂ of a polarity opposite to that of the current I₁. If the resistance values R₁ and R₂ are equal, the currents I₁ and I₂ have equal amplitudes. The current I₂ is supplied to the resistor 222.

It should be noted that similar equations to those discussed above for current source 225 lead to the following relationship:

VF2 = Vc = - VF1 ..... (10).

It should be noted that, since it is controlled by the opposite polarity and the same amplitude of the current I2, the voltage VF2 has the identical voltage value and opposite polarity with respect to the voltage VF1 in order to define the dead band.

The current sources 225 and 226, the inverting amplifier 248 and the reference voltage source 257 constitute a dead band control circuit for controlling the width of the dead band, namely, according to the level of the envelope signal SE. In the illustrated embodiment, the diodes 217 and 218 of the dead band circuit 223 have the same characteristics as those of the temperature compensation diode 256 in the reference voltage source 257. Since a closed loop is provided in the crossing signal elimination circuit 222 for controlling the width of the dead band, the temperature characteristics of the diodes 217 and 218 can be compensated stably and effectively for practical use.

As can be seen from this, the illustrated embodiment extracts the crossing signal included in the focus error signal St from the sample and hold circuit 216 and controls the width of the dead band by adjusting the voltages VF1 and VF2. Therefore, the crossing signal can be effectively eliminated or absorbed from the focus error signal to obtain a waveform as shown in Figure 9(e).

As shown in Figures 10 and 11, the deadband circuit 223 has input/output characteristics with variable Dead band width by changing the voltages VF1 and VF2, where the input/output voltage difference (VH - VFE) from zero takes volts as the center potential corresponding to the crossing signal. Since a closed loop for controlling the width of the dead band is formed in the entire crossing signal elimination circuit 220, the temperature characteristics of the diodes 217 and 218 can be compensated with respect to the level. The input/output characteristics of the dead band circuit 223 in which the voltages VF1 and VF2 define the dead band variation, as a result, determine the conditions under which the held voltage VH of the sample and hold circuit 216 is used.

Since the voltages VF1 and VF2 have opposite polarities and the same magnitude/magnitude of voltage difference with respect to the held voltage VH, and since the width of the dead band depends on the level of the crossing signal superimposed on the focus error signal, the variation of the crossing signal can be removed from the focus error signal and the influence of the crossing signal on the focus servo system can be successfully cancelled. This can be seen from the waveform of Figure 12.

In a practical design, the focus control circuit is designed to have a gain as high as possible while preventing oscillation of the focus servo circuit. The dead band circuit is designed to prevent the focus servo circuit from oscillating by adjusting the width of the dead band. This makes it easier to adjust the gain of the focus control circuit.

It should be noted that although the illustrated embodiment may have resistors 221 and 222 used as described, which have the same resistance values as the resistors 251 and 261 of the voltage sources 225 and 226, it is possible to choose the resistance values to be different from each other or to send different current amplitudes through the resistors 221 and 222, with the result that the dead band is offset with respect to the held voltage VH.

Figure 13 shows another embodiment of a crossover signal elimination circuit for an optical disk recording and playback device. The crossover signal elimination circuit 300 includes an amplifier 311 having a voltage follower structure and receiving the focus error signal St. The crossing signal elimination circuit 300 further includes a peak hold circuit 318 and a base hold circuit 319. The circuit 318 includes a diode 312, a resistor 314 and a capacitor 316. The circuit 319 includes a diode 313, a resistor 315 and a capacitor 317. The outputs of the peak and base hold circuits 318 and 319 are connected to an adder circuit 322 having resistors 320 and 321 and to an operational amplifier 323. Diodes 312 and 313, arranged in opposite polarity, are connected to respective constant voltage sources which supply respective voltages -Vcc and +Vcc through resistors 314 and 315. The voltages -Vcc and +Vcc supplied through resistors 314 and 315 supply appropriate forward currents through diodes 312 and 313. The resistance values of resistors 320 and 321 are set at the same value.

The focus error signal St is supplied to the diodes 312 and 313 of the peak hold and base hold circuits 318 and 319 via the voltage follower type amplifier 311. The peak value of the focus error signal St is rectified by the diode 312 and thereby charges the capacitor 316. The base value of the focus error signal St is rectified by the diode 313 and charges the capacitor 317.

Assume that the time constant of the resistor 314 and the capacitor 316 and of the resistor 315 and the capacitor 317 are equal = T and that this time constant T is longer by a sufficient amount than the period of the crossing signal SM2. The voltages at the terminals of the capacitors 316 and 317 then vary respectively in accordance with the held peak voltage e1 and the held base voltage e2 as shown in Figure 14(a). Since the voltage e1 and the voltage e2 are supplied through the resistors 320 and 321 having the same resistance, the voltage e at the intersection becomes (e1/2 + e2/2). This is essentially equal to the pure focus error signal ef, as shown in Figure 14(b).

Figure 15 shows a modification of the crossing signal elimination circuit of Figure 13. In this modification, the crossing signal elimination circuit has 300' operational amplifiers 330 and 331 in the peak and base hold circuits 318 and 319. These operational amplifiers 330 and 331 are provided for improving the non-linear characteristics of the diodes 312 and 313. For this purpose, the operational amplifiers 330 and 331 are arranged between the amplifier 311 and the diodes 312 and 313 of the circuits 318 and 319.

The modified circuit of Figure 15 further includes a bypass 332 for bypassing the crossing signal elimination circuit 300', namely for supplying the focus error signal St directly to the phase compensation circuit (not shown). A switch 333 is provided in the bypass circuit 332 for forming and disabling the bypass circuit. The switch 333 is controlled by a gate signal of an OR gate 336. The OR gate 336 is connected to one input terminal 334 to which an indication signal indicating the presence of the tracking operation state is supplied. The OR gate 336 is also connected to another input terminal 335 to which an indication signal of the focus adjustment operation is supplied. The OR gate 336 is responsive to a tracking operation indication signal and a focus adjustment operation indication signal to set the switch 333, namely to close it and form the bypass circuit 332.

It should be noted that the bypass circuit 332, with the switch 333 and the OR gate 336 for controlling the switch position, serves as a control circuit equivalent to the control circuit 210 shown in Figure 5.

In the embodiments of Figures 13 and 15, therefore, the influence of the crossing signal can be successfully managed by holding the peak and base values and obtaining the average values thereof.

Claims (20)

1. Apparatus for recording and/or reproducing from an optical disc, comprising: an optical head (3) for scanning a light beam across an optical disk (1) having a plurality of generally circumferential (circularly running) grooves (G) to reproduce an information signal including a focus error signal (St); a focus servo system (7) having a focus actuator (73) operable to move an objective lens of the optical head (3) to focus a light beam on the optical disk (1); and means (71) associated with the focus actuator (73) for deriving a focus control signal (SFD) based on the focus error signal (St) and for controlling the focus actuator (73); marked by, Means (11; 220; 300) active in the access mode of the device in which the light beam is displaced across at least one of the grooves (G) for eliminating a signal component (SM1) modulated by said groove (G) and superimposed on the focus error signal (St), said means for eliminating the signal component having a deadband circuit (112; 223) and a sample and hold circuit (116; 216), said deadband circuit defining a deadband for the focus error signal (St) for absorbing fluctuations of said focus error signal within the deadband, whereby a value held in said sample and hold circuit is unchanged. 2. Apparatus according to claim 1, comprising a switch device (119,330) associated with the means (11,220,300) for eliminating said signal component for switching the operating state of said means for eliminating the signal component, the switch device being responsive to an indication signal relating to a non-tracking operating state to activate said means for eliminating the signal component. 3. Device according to claim 2, wherein said switch device (119,330) is operable to render said means (11;220;300) for removing the signal component inactive during the focus search operation. 4. Apparatus according to claim 1, claim 2 or claim 3, comprising means (233-266) for adjusting the dead band. 5. Device according to claim 4, wherein said means (233-266) for adjusting the dead band are operable to detect the level of the signal component (SM1) for adjusting the width of said dead band in dependence thereon. 6. Apparatus according to claim 5, wherein said means (233-266) for adjusting the dead band are operable to detect said signal component (SM1) superimposed on an output signal of the sample and hold circuit (216) for adjusting said dead band. 7. Apparatus according to any preceding claim, comprising means for forming a signal bypass path of said deadband circuit (112; 223) to directly supply the input focus error signal (St) to the sample and hold circuit (116), said signal path including a switch (119) to be switched between a conducting state to form said path and a non-conducting state to interrupt said path. 8. Apparatus for recording and/or reproducing an optical disc, with: an optical head (3) for scanning a light beam across an optical disk (1) having a plurality of generally circumferential grooves (G) to reproduce an information signal including a focus error signal (St); a focus servo system (7) having a focus actuator (73) operable to move an objective lens of the optical head (3) to focus a light beam on the optical disk (1); and Means (71) associated with the focus actuator (73) for to derive a focus control signal (SFD) based on the focus error signal (St) and to control the focus actuator (73); marked by; Means (11; 220; 300) active in the access mode of the device in which the light beam is displaced across at least one of the grooves (G) to eliminate a signal component (SM1) modulated by this groove (G) and superimposed on the focus error signal (St), said means for eliminating the signal component having a peak value holding circuit (318) for holding a peak value of the focus error signal (St), a base value holding circuit (319) for holding the base value of this focus error signal (St) and an adder circuit (322) for adding the output signals of the peak value and base value holding circuits (318, 319). 9. Apparatus according to claim 8, wherein each of the peak and baseline holding circuits (318,319) comprises diodes (312,313) and has means (330,331) for compensating for non-linear characteristics of these diodes (312,313). 10. Device according to claim 8 or 9, with means for forming a signal path as a bypass for the holding circuits (318, 319) for peak value and base value in order to supply the input focus error signal (St) directly to the output, this signal path having a switch (333) which is to be switched between a conducting state for forming this path and a non-conducting state for interrupting this path. 11. Device according to claim 7 or 10, in which this switch (119,333) is to be switched to the conductive state during an operating state of the tracking of the device. 12. Device according to claim 7, 10 or 11, in which this switch (119; 333) is to be switched on during an operating state of the focus adjustment. 13. A focus control system for an optical disk recording and/or reproducing apparatus, including: an optical head (3) for scanning a light beam across an optical disk (1) having a plurality of generally circumferential grooves (G) for reproducing an information signal including a focus error signal (St), and a focus servo system (7) having a focus actuator (73) operable to move an objective lens of the optical head (3) to focus a light beam on the optical disk (1), the focus control system comprising: a focus control signal generator (71) associated with the focus actuator (73) for deriving a focus control signal (SFD) based on the focus error signal (St) and controlling the focus actuator (73); marked by: Means (11; 220) for absorbing a crossing signal component, wherein said means are arranged before said focus control signal generator (71) for absorbing fluctuations of said focus error signal (St) within a predetermined fluctuation range to eliminate a crossing signal (SM1) superimposed on said focus error signal (St), wherein said means for absorbing said signal component comprise a deadband circuit (112, 223) and a sample and hold circuit (116; 216), and said deadband circuit sets a deadband for said focus error signal (St) for absorbing fluctuations of said focus error signal lying within said deadband, a value held in said sample and hold circuit being unchanged. 14. A focus control system according to claim 13, comprising means (233-266) for adjusting the deadband. 15. Focus control system according to claim 14, wherein said means (233-266) for adjusting the dead band are operable to detect the level of the signal component (SM1) for adjusting the width of said dead band in dependence thereon. 16. A focus control system according to claim 15, wherein said means (233-266) for adjusting the dead band are operable to detect said signal component (SM1) superimposed on an output signal of the sample and hold circuit (216) for adjusting said dead band. 17. A focus control system according to any one of claims 13 to 16, comprising means for forming a signal bypass path of said deadband circuit (112; 223) for feeding the input focus error signal (St) directly to the sample and hold circuit (116), said signal path including a switch (119) to be switched between a conductive state for forming said path and a non-conductive state for interrupting this path and for switching the conductive state depending on an indicating signal of one of the tracking and focus adjustment states. 18. A focus control system for an optical disk recording and/or reproducing apparatus, said apparatus including an optical head (3) for a light beam for scanning an optical disk (1) having a plurality of generally circumferential grooves (G) for reproducing an information signal including a focus error signal (St), and a focus servo system (7) having a focus actuator (73) operable to move an objective lens of said optical head (3) to focus a light beam on said optical disk (1), said focus control system comprising: a focus control signal generator (71) associated with the focus actuator (73) for deriving a focus control signal (SFD) based on the focus error signal (St) and controlling the focus actuator (73); marked by: a smoothing means (300) for the focus error signal, said means being arranged before the generator (71) for the focus control signal, for smoothing the focus error signal (St) so as to produce a crossing signal (SM1) superimposed on the focus error signal (St). wherein said means (300) for smoothing the focus error signal has a peak value holding circuit (318) for holding the peak value of said focus error signal (St), a base value holding circuit (319) for holding the base value of said focus error signal (St), and an adding circuit (322) for adding the output signals of said peak value and base value holding circuits (318,319). 19. A focus control system according to claim 18, wherein each of the peak and baseline hold circuits (318,319) comprises diodes (312,313) and has means (330,331) for compensating for non-linear characteristics of these diodes (312,313). 20. Focus control system according to claim 18 or 19, with means for forming a signal path as a bypass for the smoothing means (300) for the focus error signal in order to supply the input focus error signal (St) directly to the output, said signal path comprising a switch (333) which is to be operated between a conductive state for forming this path and a non-conductive state for interrupting this path and for switching on this conductive state depending on an indicating signal of an operating state of the tracking and the focus adjustment.